Fix MQTT sample config

plugins/outputs/mqtt/README.md

@@ -2,13 +2,13 @@
 
 This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt Producer.
 
-```
+```toml
 [[outputs.mqtt]]
   ## URLs of mqtt brokers
   servers = ["localhost:1883"]
 
   ## topic for producer messages
-  topic = "telegraf"
+  topic_prefix = "telegraf"
 
   ## QoS policy for messages
   qos = 2
@@ -32,8 +32,6 @@ This plugin writes to a [MQTT Broker](http://http://mqtt.org/) acting as a mqtt
 
   ## Data format to output.
   data_format = "influx"
-
-
 ```
 
 ### Required parameters: